1. Field of the Invention
The present invention relates to optical amplifiers, and particularly to an optical amplifier including a variable optical attenuator and an external attenuating medium connected in series.
2. Description of the Related Art
In optical wavelength multiplex transmission systems, an erbium-doped fiber amplifier (EDFA) is generally used as an optical amplifier used for a transmitter or a repeater. The EDF optical amplifier has a set signal gain determined by the signal gain characteristics of the EDF (such as dependence of gain on the wavelength). Accordingly, the amplifier includes a variable optical attenuator (VOA) for absorbing changes in amplifier gain and interstage losses such as an insertion loss of a dispersion compensating optical fiber (DCF) (refer to Japanese Unexamined Patent Publication Nos. 2004-72062 and H10-51057). When the amplifier gain or interstage loss decreases, the loss by the VOA is increased by the amount of decrease.
The demand for optical amplifiers having a wide dynamic range (7 to 16 dB, for instance) has been growing to keep up with a wide variety of user requirements for system gain. The VOA loss range must be increased accordingly, but a great VOA loss range would worsen the noise factor. To avoid the problem, two stages of VOAs are configured.
FIG. 13A is a block diagram of a conventional optical amplifier. The shown optical amplifier has two stages of VOAs. Signal light is input to an input terminal 201 shown in FIG. 13A. The input signal light is output through a coupler 202 to an EDF 204. The coupler 202 combines the signal light with pumped light from a laser diode (LD) 203 and outputs the combined light to the EDF 204. Now, the signal light has a gain depending on the power of the pumped light. A VOA 205 attenuates the signal light output from the EDF 204. A coupler 206, an LD 207, and an EDF 208 are analogous to the coupler 202, the LD 203, and the EDF 204. Couplers 212 and 216, LDs 213 and 217, EDFs 214 and 218, and a VOA 215 are analogous to the corresponding elements between the input terminal 201 and an output terminal 209. Amplified signal light is output from an output terminal 219. A DCF 210 is connected to terminals 209 and 211 and compensates for wavelength dispersion of the signal light output from the EDF 208.
FIG. 13B shows the level of signal light varying in different stages of the optical amplifier shown in FIG. 13A. The height of waveforms A and B in FIG. 13B represents the level of light (in units of dBm, for instance), and vertical dotted lines are separators between different stages of the optical amplifier shown in FIG. 13A.
If the level of signal light input to the input terminal 201 is originally represented by waveform A and increased as represented by waveform B, the gain of the EDF 204 decreases to reduce the level of signal light (the inclination of waveform B is smaller than the inclination of waveform A). Because the total gain of the EDF 204 and the EDF 208 must be kept constant because of the EDF signal gain characteristics, the gain of the EDF 208 needs to be increased by the amount of decrease in gain of the EDF 204. (The inclination of waveform B becomes greater than the inclination of waveform A, between the input and output of the EDF 208.) Squares or circles in the figure represent that the marked segments have the same inclination, which means that the total gain of the EDF 204 and the EDF 208 is kept constant.
The level of light at the terminal 209 must also be kept constant, so that the amount of loss (attenuation) by the VOA 205 becomes as represented by waveform B, which is greater than that of waveform A. The same configuration is provided between the terminal 211 and the output terminal 219. The VOA loss range collectively provided by the VOA 205 and the VOA 215 prevents the noise factor from becoming worse.
When a change in interstage loss (change in loss across the DCF 210) becomes 10 dB or greater, the loss range of the VOA 215 would increase in the configuration shown in FIG. 13A.
Suppose that the amount of loss in level of light between the input and output of the DCF 210 becomes as represented by waveform B, which is smaller than that represented by waveform A (the inclination of waveform B becomes smaller than the inclination of waveform A). In other words, suppose that the level of signal light input to the terminal 211 increases. The gain of the EDF 214 would decrease because of the upper limit of power of the LD 213, then the gain of the EDF 218 should be increased under constant sum gain control. Because a constant (target) level of signal light must be obtained at the output terminal 219, the amount of loss by the VOA 215 increases by the amount of decrease in loss by the DCF 210. This would worsen the noise factor, depending on the amount of decrease in power input to the EDF 218, and the LD 217 should have a great power. To avoid this problem, the VOA and the DCF are connected in series.
FIG. 14A is a block diagram of an optical amplifier in which a VOA and a DCF are connected in series. In the optical amplifier shown in FIG. 14A, a VOA 229 and a DCF 231 are connected in series. The DCF 231 is connected to terminals 230 and 232. Couplers 222 and 226, LDs 223 and 227, EDFs 224 and 228, and a VOA 225 are analogous to the couplers 202 and 206, the LDs 203 and 207, the EDFs 204 and 208, and the VOA 205 in FIG. 13A. In the optical amplifier shown in FIG. 14A, the single VOA 225 absorbs the dynamic range. No VOA is connected after the DCF 231, between an EDFA including a coupler 233, an LD 234, and EDF 235 and another EDFA including a coupler 236, an LD 237, and an EDF 238. Signal light is input to an input terminal 221 and output from an output terminal 239.
FIG. 14B shows the level of signal light varying in different stages of the optical amplifier shown in FIG. 14A. The height of waveforms A and B in FIG. 14B represents the level of light, and vertical dotted lines are separators between different stages of the optical amplifier shown in FIG. 14A.
The operation from the input terminal 221 to the EDF 228 is the same as the operation of the amplifier shown in FIG. 13. If the amount of loss by the DCF 231 becomes as represented by waveform B, decreased from the amount represented by waveform A, the amount of loss by the VOA 229 is increased by the amount of decrease so that the level of signal light is kept constant at the terminal 232. The noise factor can be prevented from becoming worse by connecting the VOA 229 and the DCF 231 in series to keep the level of light after the DCF 231 constant.
The DCF 231 is directly connected and disconnected by the user. The connection or disconnection of the DCF 231 must be confirmed by checking the amount of loss before and after the DCF 231. Accordingly, an optical detection section for detecting the level of light, such as a photo diode (PD), must be provided before and after the DCF 231.
FIG. 15 is a block diagram of an optical amplifier which can detect the connection or disconnection of a DCF. In the shown optical amplifier, signal light input to an input terminal 241 passes one EDF 246, a DCF 253, and another EDF 259 and is output from an output terminal 262.
Couplers 242 and 247 branch signal light into a PD 243 and a PD 248 respectively, and the PD 243 and the PD 248 convert the light to an electric signal. An AGC 271 adjusts the pumped light of an LD 245 in accordance with the electric signals of the PD 243 and the PD 248, or the levels of signal light before and after the EDF 246. The pumped light is input through a coupler 244 to the EDF 246. Couplers 255, 257, and 260, PDs 256 and 261, an LD 258, and an AGC 273 after the DCF 253 function in the same way. The AGC 271 and the AGC 273 bring the signal light to a target level at the output terminal 262. An interstage loss control block 272 controls a VOA 249 to keep the total gain of the EDF 246 and the EDF 259 constant.
The interstage loss control block 272 monitors the level of light before and after the DCF 253, in accordance with the level of light branched by a coupler 250 and detected by a PD 251 and the level of light branched by the coupler 255 and detected by the PD 256. Then, the interstage loss control block 272 detects a disconnection (of the DCF 253 from terminals 252 and 254) or a connection (of the DCF 253 to the terminals 252 and 254), in accordance with the level of light before and after the DCF 253.
To detect the connection or disconnection of an external attenuating medium (in this example, the DCF 253) to be connected or connected in series with a variable optical attenuator (in this example, the VOA 249), an optical detection section (in this example, the PD 251 and the PD 256) must be provided before and after the external attenuating medium. If the optical detection section is provided before the variable optical attenuator and after the external attenuating medium, the varying amount of attenuation by the variable optical attenuator makes it impossible to check the connection or disconnection of the external attenuating medium in accordance with the correct amount of loss by the external attenuating medium.
The detection section provided before and after the external attenuating medium, however, requires that the signal light is branched into the detection section, and the loss depending on the branching ratio would lead to SN degradation.
For instance, the PD 251 provided before the DCF 253 shown in FIG. 15 would provide a loss depending on the branching ratio and cause SN degradation accordingly.
To prevent SN degradation, signal light must be amplified by the amount of loss depending on the branching ratio, and a higher LD power is required. This would reduce the cost effectiveness and would increase power consumption because of LD temperature control.